1. Field of the Invention
The invention concerns flat lapping machines and lapping beds for such machines.
2. Description of the Prior Art
Very flat surfaces of high quality are usually produced by lapping, a machining operation in which material is removed by means of an abrasive such as silicon carbide, alumina, diamond, etc., in suspension in a fluid such as water, oil, kerosine, etc., deposited onto a surface (bed) which is moved relative to the surface of the parts to be lapped. The movement of the abrasive particles relative to the surface of the parts removes material and the suspension liquid, to which a lubricant liquid to favor cutting may be added, removes the waste material, reduces the direct friction between the surface of the bed and the surface of the parts, and acts as a coolant. Pressure is applied to the opposite side of the parts to be lapped to control the rate at which material is removed.
The quality of the flatness achieved at the surface of the parts is directly related to the flatness of the lapping bed and to the ratio between the diameter of the parts and the diameter of the lapping bed.
Flat lapping machines are so constructed that the path of movement of the parts on the surface of the bed is the resultant of two circular movements: rotation of the bed about its axis, and rotation of the parts about axes perpendicular to the surface of the bed and offset relative to its center.
The parts are placed loose in bulk or in circular plates incorporating cells, inside rings usually of metal which confine their movement on the surface of the bed. A metal disk rests on the other side of the parts and transmits the force applied by a piston-and-cylinder actuator or by weights to the parts. A sheet of felt may be provided between the metal disk and the parts.
The rings are distributed circumferentially about the axis of the bed at equal distances from its axis. There are usually three or four rings, their axes being fixed relative to that of the bed and on radii spaced by 120.degree. for three rings or 90.degree. for four rings.
The rings can rotate freely, as a result of the resultant force due to the differential friction forces generated by the relative linear speeds of the parts relative to the center of the bed, or by being driven by a motor or by a toothed wheel driven by the bed and meshing with teeth cut into the perimeter of the rings.
In the case of fine lapping of parts made from hard materials such as ceramics, where the flatness required is in the order of 0.6 .mu.m or better and the roughness required is in the order of 0.25 .mu.m Ra or better, the lapping bed has a spiral groove of constant pitch and geometry on its surface to enable rapid evacuation of the waste material and so prevent the formation of a thick film between the parts and the bed which could compromise flatness.
Various machines commercially available worldwide use the principles described above. Examples of such machines are SPEEDFAM, PETER WOLTERS, and STAHLI.
It will be appreciated that with this kind of arrangement the correct disposition of the parts within the rings enables the parts to sweep over all the surface of the bed so that the latter tends to be worn down equally in all parts.
This is satisfactory if extremely precise flatness is not required but proves to be inadequate when precise flatness is required in the manufacture of very large numbers of parts. In this case, it is necessary to maintain the flatness of the lapping bed and to compensate for differential wear phenomena to which it is subject and which results in hollowing out of the bed.
Various methods are used to prevent irregular wearing away of the bed. They include reversing the direction of forced rotation of the rings relative to the surface of the bed at regular intervals; prestressing the bed so that its surface bulges, which prestressing is released as the bed is worn down; and variable positioning of the rotation axis of each ring relative to the center of the bed.
All these methods have been found to be somewhat ineffective and, most importantly, to compromise productivity. Although differential wearing of the bed can be partially compensated or slowed down, this is achieved as the result of costly operations which waste time and require highly qualified personnel to inspect the bed at regular intervals.
An object of the invention is to alleviate the abovementioned disadvantages by means of a bed geometry which is designed to wear away regularly under the conditions of use explained above.